In cellular networks, it is known to assign resource elements of the available radio capacity to be used for data transmission to or from a user equipment (UE). Specifically, such resource elements may be organized in a time-frequency grid.
For example, the LTE (Long Term Evolution) radio technology specified by 3GPP (3rd Generation Partnership Project) uses Orthogonal Frequency Division Multiplexing (OFDM) for downlink (DL) transmissions to UEs and Discrete Fourier Transform (DFT) spread OFDM, also referred to as Single Carrier (SC) OFDM, for uplink (UL) transmissions from the UEs. In this case, the available resources may be organized in a time-frequency grid of subcarriers with 15 kHz width and time elements corresponding to the duration of one OFDM symbol. A resource element may then extend over one subcarrier in the frequency domain and the duration of one OFDM symbol in the time domain. Such a time-frequency grid may be defined individually for each antenna port.
In the time domain, LTE DL transmissions are organized in radio frames of 10 ms duration, each radio frame consisting of ten equally-sized subframes of 1 ms duration, also referred to as TTI (Transmission Time Interval). The subframes are in turn divided into two slots, each having 0.5 ms duration. Each subframe includes a number of OFDM symbols which may be used for conveying control information or data.
The resource allocation in LTE is accomplished on the basis of resource blocks. A resource block corresponds to one slot in the time domain and 12 contiguous subcarriers in the frequency domain. In LTE, the highest granularity level of assigning resource elements corresponds to two in time consecutive resource blocks, also referred to as a resource block pair or Physical Resource Block (PRB). A PRB thus extends over the entire time duration of the subframe. Dynamic scheduling may be performed in each subframe. For this purpose, an LTE base station, referred to as evolved Node B (eNB) may use a DL control channel, e.g., the Physical DL Control Channel (PDCCH) to transmit DL assignments and UL grants to the UEs served by the base station. The PDCCH is transmitted in the first OFDM symbol(s) of the subframe.
If an UE has decoded such DL assignment, it knows which time and frequency resources in the subframe contain DL data destined to the UE. Similarly, upon receiving an UL grant, the UE knows on which time/frequency resources it should transmit UL data. The DL data are carried by a channel which is shared by the UEs served by the base station and is referred to as Physical DL Shared Channel (PDSCH). Similarly, the UL data are carried by a channel which is shared by the UEs served by the base station and is referred to as Physical UL Shared Channel (PUSCH).
In the above-mentioned LTE radio technology, but also in other radio access technologies, demodulation and decoding of sent data typically requires estimation of a propagation characteristic of the radio channel. This may be accomplished by using transmitted reference symbols (RS), i.e., symbols known by the receiver. In the LTE radio technology, cell specific RS (CRS) are transmitted in all DL subframes. Besides their usage for DL channel estimation, they may also be used for mobility measurements performed by the UEs. In addition, also UE specific RS (also referred to as DMRS) may be used.
To allow for more efficient channel estimation, the LTE radio technology also provides a concept referred to as physical resource block (PRB) bundling. In these concepts, a group of frequency consecutive PRB pairs within a subframe are grouped to a PRB bundle. For such PRB bundle, the receiver can assume that precoding at the transmitter remains static so that the DMRS are not significantly affected. This allows the receiver to perform channel estimation by averaging over the DMRS of the entire PRB bundle, rather than just a PRB pair.
A basic unit of transmission in the LTE radio technology is the transport block. In each TTI, one or two transport blocks may be transmitted to each scheduled user. Reception of a transport block may either succeed or fail.
Each transport block is divided into one or more code blocks. Each code block is separately encoded by the transmitter and decoded by the receiver using an error correcting (channel) code. If a code block is not decoded without errors, the reception of the entire transport block to which the code block is associated is considered as failed. In such case, a retransmission of the entire transport block is triggered.
The system selects several parameters associated with the transmission of a transport block. These parameters include modulation and coding scheme (MCS), code-rate as well as the number of spatial streams, or layers, onto which the transport block should be mapped. The selection of these parameters allows for achieving a trade-off between transmission reliability and efficiency of resource usage, as selecting the parameters too conservatively may lead to over-use of radio-resources while selecting them too aggressively may lead to failed reception of the transport block. The system may dynamically set the parameters to achieve that only a small portion of the received transport blocks fails to be decoded. The parameters can however only set with the granularity of a transport block. Adjusting the parameters between two code blocks within the same transport block is not possible. A reliable transmission of a transport block therefore requires a minimum decoding performance for each code block within the transport block. If one code block does not meet this minimum requirement, the transmission of the entire transport block fails, even if other code blocks have a significantly better decoding performance.
In the LTE radio technology, the code blocks are mapped onto the resource elements consecutively symbol by symbol in a frequency-first manner, over increasing subcarrier indices starting with the first OFDM symbol after the control region and successively continuing over all OFDM symbols in the subframe, as for example specified in 3GPP TS 36.211 V11.3.0 (2013-06). That is to say, the next available OFDM symbol on the time axis is only selected after for the present OFDM symbol all subcarriers allocated to a certain UE have been mapped. This allows for assuming that every code block is subjected to similar channel conditions. Further, this allows for starting decoding of the code blocks already before the entire subframe is received.
Two fundamental functions that need to be performed by an LTE receiver are channel estimation and MIMO (Multiple Input/Multiple Output) equalization. These functions require a significant amount of arithmetic operations. Accordingly, it is beneficial to distribute these functions over several parallelization instances that in turn map to digital signal processors, software threads, or hardware accelerators. This parallelization may be accomplished in such a way that each parallelization instance processes data that relates to a specific part of the received bandwidth. That is to say, the parallelization may be performed in the frequency domain. Further, the parallelization may consider the PRB bundles, so that each PRB bundle is assigned to only one parallelization instance. This facilitates the implementation of channel estimation and MIMO equalization.
Parallelization of the decoding process is however more complex. In particular, the above-described mapping may cause a code block to be distributed over resource elements which are located in different PRB bundles and therefore handled by different parallelization instances performing channel estimation and MIMO equalization. Decoding of the code block therefore requires information from these different parallelization instances, which means that the decoding task cannot be straightforwardly assigned to one of these parallelization instances.
Further, due to the above-described mapping of the code blocks to the resource elements it may occur that one code block is mapped to a relatively small contiguous part of the allocated bandwidth. This is specifically true when using a large number of MIMO layers, e.g., more than four, and/or a high-order MCS, which means that more data symbols can be mapped to the same subcarrier. This is not optimal from a frequency diversity point of view, and may adversely affect the decoding performance. Further, if the channel quality varies over frequency this also causes some code blocks to experience worse channel conditions than others. As mentioned above, this may result in failed transmission of the entire transport block.
Further, problems may occur if a given OFDM symbol encounters broadband-interference, e.g., due to CRS transmission in a neighboring cell. In such cases, multiple data symbols would be affected simultaneously due to the code block being very localized in the time domain. This typically makes decoding impossible. For other code blocks, which are transmitted on another OFDM symbol, the broadband interference may be absent, which then causes a severe imbalance in the decoding performance of the code blocks. Similar considerations apply when certain OFDM symbols are intentionally punctured.
Still further, problems may occur in connection with the HARQ (Hybrid Automatic Repeat Request) mechanism utilized by the LTE radio technology, which has a tendency to map the code blocks of a retransmitted transport block to the same resource elements of the subframe which were also used for the failed initial transmission, provided that the PRB allocation to the UE is the same for the initial transmission and the retransmission. Accordingly, there is an increased likelihood that the retransmission fails for the same reasons as the initial transmission.
Moreover, the mapping of code blocks to resource elements as used in the LTE radio technology has the effect that one code block may span over several consecutive PRB bundles. If parallelization of the channel estimation and MIMO equalization is implemented as described above on a PRB bundle basis, the output from multiple parallelization instances is needed for decoding the code block, which significantly complicates decoding.
Accordingly, there is a need for techniques which allow for efficient transmission of data transmission over a radio interface using subframes with a plurality of resource elements organized in a time-frequency grid.